Effectively yes. They sold to Salesforce for $30B, but they easily could have won the market if not for this move on Microsoft’s part.
There is no could have been in reality. Laws of physics was not broken since Slack was invented. You are reducing a complex system of economics, technology, market, phycology, politics etc into a sentence "could have been". It sounds smart and logical but it has no existential relevance.
hopefully this time, the DOJ is not merciful and breaks Microsoft into 3-4 different entities.